The car is a 2200lb street toy/weekend driver/occasional rallycross (everything I own is occasional rallycross) kind of deal. Want to eliminate the power brakes in the future. I already redrilled the brake pedal for a 7:1 ratio as future-proofing since it is way easier to get the pedal out now before putting the engine back in.

The sticking point is that I'd prefer to use a single dual-stage master cylinder rather than two masters and a bias bar, for simplicity reasons. Yes it's leaving stuff on the table, but it's not that kind of car. This is where my head starts to hurt math-wise.

I'm probably going to be using 1 3/8 4-piston calipers on the front, and 1 3/8 sliding calipers on the rear. This is less front pressure than the stock 2" sliders but I've found these cars seem to "like" a lot less front brake so I'm going to roll with it.

The part where I go crosseyed is that, okay, .7" front master .75" rear on a twin master setup. So the pedal is pushing on the masters in parallel. With a dual-stage, the pedal is pushing on them in series, so would that not mean that pressure is doubled for a given pedal force? So I wouldn't necessarily want a .7-.75" master, I'd want something with about double the bore area, which my math says is about 1" diameter? That seems... big. Especially given that I have a similar car with a .875" dual stage and it really, really needs power brakes, and I don't see its 4.5:1 pedal ratio making that huge of a difference... or would it?

Conveniently, I've found a .75" bore, manual brake, tandem master cylinder that is very swappable. The math says it would suck. Gut feeling says it'd be okay. I don't like it when the math doesn't agree with gut feeling, that means either the math is wrong or my gut feeling is wrong and either way it's not confidence inspiring.

I don't see why the pressure would double. You're talking about a single bore with two pistons in it right?

Assuming that nothing changes but the arrangement of the cylinders (IE: same calipers, same master cylinder piston volumes) I don't see how it'd be different.

As far as the pedal ratio thing, I'm pretty sure a 7:1 ratio is a 55% increase in mechanical advantage over a 4.5:1 ratio. So with the same force pedal side you'd have 55% more force master cylinder side. I'm sure you know you're going to gain a good bit of pedal travel to get this ratio.

I've not worked these kinds of problems before, so hopefully someone will let me know if I'm missing something.

If using two pistons to build pressure, the force from the pedal is halved (assuming 50/50 on a bias bar) to each MC. This is the same for a tandem or dual MC setup. A word of caution on the tandem MC, some have a spring between the 1st and second piston. This will cause the circuit closer to the pushrod to gain pressure faster than the one at the end of the MC. Ran into this issue with the Volvo 240 MC. Super odd brake feel at times.

On the volvo with un-powered brakes (blew out the booster and didn't have a spare), the brakes were hard and not desirable, but it worked well enough for a few podiums. New brakes are in the future for us.

That setup was RX7 front calipers (4-piston @ 36mm/1.42in) and stock volvo 240 rear (2-pistons @ 38mm/1.5in), and stock volvo 240 tandem MC with 5/8in bore to the front and 7/8in bore to the rears. Pedal ratio was just a hair over 5:1.
Basically, don't do that ^

Better solution would have been to remove the spring in the MC and make a spacer, and then replace the crappy booster with a good one.

Edited 1 time(s). Last edit at 09/05/2017 12:12PM by Robert Culbertson.

QuoteRobert CulbertsonBetter solution would have been to remove the spring in the MC and make a spacer, and then replace the crappy booster with a good one.

I think putting a spacer in will make things screwy too. I'll check a little more thoroughly later but I think you'll end up getting different brake pressure in the different circuits. I'd say either remove the spring or add a second one with the same rate to the other cylinder.

Edit: I think you're going to get the same pressure unless you bottom out the piston/spacer/piston.

QuoteRobert Culbertson
You'd get different pressures, but it would be consistent. Basically like running a balance bar.

If you don't mind explaining, I don't understand. As far as I can tell all the spacer will do is reduce the volume inside the cylinder for that circuit. Pressure would stay the same because the piston surface area doesn't change, until you run out of volume (IE: spacer is touching both pistons). What am I missing?

Quotejodo257
I don't see why the pressure would double. You're talking about a single bore with two pistons in it right?

Doubled relative to using a bias bar and two master cylinders next to each other. (Well, relatively doubled)

My interpretation of a tandem master is that it's just a regular single master cylinder with a divider in the middle to keep two halves of the system separate if one blows out. Certainly when I have replaced jam-jar (pre 1968 braking) masters with tandem masters, I've kept the bore the same and everything felt the same.

So I spoke with one of my engineering professors. A tandem master cylinder with a single bore and no springs will develop the same pressure in each circuit (which will be the force applied divided by bore area - not half the force.)

A double master cylinder cylinder setup will halve the force going to each piston so you'll want to halve the bore area to get equivalent pressure.

Quotejodo257
So I spoke with one of my engineering professors. A tandem master cylinder with a single bore and no springs will develop the same pressure in each circuit (which will be the force applied divided by bore area - not half the force.)

A double master cylinder cylinder setup will halve the force going to each piston so you'll want to halve the bore area to get equivalent pressure.

Hopefully I'm not missing something.

Your professor is Right..

Just think of a real world example--Plenty of cars followed the lead of safety pioneers Mercedes and Saab and Volvo and introduced "dual diagonal" systems with one circuit LF and RR and the other circuit RF and LR..

On the nicests of those cars, the lightest and the ones with the best brakes the master cylinder is tandem, Lockheed to 71 and ATE thereafter.
Same bore the whole way..
IF there was some devine intervention and The Gods somehow created less pressure at the master on one outlet vs the other, then how could the car brake straight?
Same sized 2" pistons on front and 5/8" or 3/4" on rears..Cars brake straight with same size stuff at the wheels---then the pressure at the ports of the master is same..

More un-sound thinking from a troubled mind.

Now Pete, if you ditch the booster--which boosts PEDAL effort what? 2 to 3.5 times IF you loose assist there, they you better get some assist somewhere...
The pistons sized you mentioned sound small..
Give you an idea..240 Volvo have 4 x 1.5" pistons front, 2 x 1.5" per caliper back..a delightful size on cars around 2050--2300lbs...That worked,,,but using the lurv-erly Sumitomo RX7 4 piston things without a booster didn't work.. and they 4 x 36mm..turns out that's nearly 19% reduction..

You're to hafta make up boost assist, That means larger pistons at master and wheel cylinders.

Quotejodo257
So I spoke with one of my engineering professors. A tandem master cylinder with a single bore and no springs will develop the same pressure in each circuit (which will be the force applied divided by bore area - not half the force.)

A double master cylinder cylinder setup will halve the force going to each piston so you'll want to halve the bore area to get equivalent pressure.

Hopefully I'm not missing something.

That's right.
But what happens when there's a spring between them and they're different bores?

If the bores are different the pressure developed is proportional to bore area and force applied. Pressure1/Pressure2 = Area2/Area1.

A spring in between the first and second piston will reduce pressure in the first circuit, and a spring between the second piston and the end of the master cylinder will reduce pressure in the second circuit.

If you're combining any of the cases add the effects.

One more thing, if you raise the piston size at the master cylinder AND the wheel cylinder so that the areas stay in proportion, you won't get a change in braking force. You'll want to increase the ratio of wheel cylinder area/master cylinder area. This will mean that'll you need to move more fluid though.

Quotejohn vanlandingham
Now Pete, if you ditch the booster--which boosts PEDAL effort what? 2 to 3.5 times IF you loose assist there, they you better get some assist somewhere...
The pistons sized you mentioned sound small..
Give you an idea..240 Volvo have 4 x 1.5" pistons front, 2 x 1.5" per caliper back..a delightful size on cars around 2050--2300lbs...That worked,,,but using the lurv-erly Sumitomo RX7 4 piston things without a booster didn't work.. and they 4 x 36mm..turns out that's nearly 19% reduction..

You're to hafta make up boost assist, That means larger pistons at master and wheel cylinders.

Allegedly the booster provides 4:1 assist. No idea at what vacuum though. Certainly with the bridge port I can run out of assist in less than one brake application. The car getting converted to manual is getting a 48mm side draft so I'm expecting it to also not do much for vacuum. Plus I don't like the idea of sourcing vacuum from half the engine, seems like it would cause odd running sometimes.

Key to using the four pots is because they are 1 3/8" bore. The same castings were available with 1 3/4" bore, and I need that extra 3/16" of meat to machine away. Convert some of the caliper to aluminum shavings and it will fit on the 9" rotor just fine and clear the OE 13" wheels. I figure that there's enough pads available for Dynalite calipers that in the off chance that I end up wanting more front braking, I can go to a grippier pad.

Just an FYI:
The Rx7 calipers work just fine with a larger diameter rotor, like a 9-series volvo. Using the 9-series rotor results in brake torque that's withing 1% of a stock volvo setup (clamping force and radius, easy things to calculate). The added benefit is the pad selection (WAY cheaper) and better heat dissipation.

That's the current setup on the rally Volvo, and there's no issues with locking up the tires at any speed...